1. Field of the Invention
This invention relates to a circuit board and, more particularly, to a circuit board suitably used as a suspension board with circuit.
2. Description of the Prior Art
In general, a circuit board, such as a flexible wiring board, comprises, as shown in FIG. 15, a base layer 101 formed of insulating material, a conductive layer 102 formed on the base layer 101 in the form of a predetermined circuit pattern, and a cover layer 103 completely covering outer portions of the circuit pattern and land portions of the base layer 101 extending between lines of wire formed in the form of the circuit pattern. These circuit boards are in wide use in the field of electronics.
In recent years, the need for transmitting large volumes of information at high speed and processing them at high speed is now being increased. To keep with this trend, circuit boards, such as a flexible wiring board, or a suspension board with circuit mounting thereon a magnetic head of hard disk drive in particular, are being required to transmit high frequency electric signals at high speed.
With increase of capacitance between lines of wire formed in the circuit board in the form of a circuit pattern, a propagation constant of the circuit increases to cause time delay in transmission speed and, as a result of this, the high frequency electric signals cannot be transmitted at high speed.
In order to reduce capacitance between the lines of wire, widening an interval between the adjoining lines of wire or reducing dielectric constant of the insulating material used for the base layer and the cover layer is conceivable. But, widening the interval between the adjoining lines of wire leads to increase of inductance and thus increase of the propagation constant and also puts restrictions on the design of the circuit pattern. On the other hand, reducing the dielectric constant has the limits on the properties of material.
It is the object of the invention to provide a circuit board, having a circuit pattern of adequate high-frequency characteristics, for transmitting the high-frequency electric signals at high speed.
The present invention is directed to a novel circuit board comprising a base layer formed of insulating material and a conductive layer formed on the base layer in the form of a specified circuit pattern, wherein at least two lines of wire of the circuit pattern are spaced apart from each other to provide an air layer therebetween.
With this construction, since the air layer is made to lie between the lines of wire, dielectric constant between the lines of wire can be reduced. As a result of this, the capacitance therebetween can be reduced to produce improved high-frequency characteristics of the circuit pattern. Hence, the circuit board having this circuit pattern can be effectively used as the circuit board for transmitting high frequency electric signals at high speed.
It is preferable that top surfaces of the respective lines of wire which are formed in the form of the circuit pattern are covered with a cover layer formed of insulating material.
This construction can provide the result of protecting the lines of wire against an external physical force applied from above the lines of wire, for example, when mounting and assembling the circuit board. Thus, the lines of wire can be prevented from being damaged by such an external force to thereby provide improved durability of the circuit board.
Further, it is preferable that the circuit pattern includes a line of wire having one side surface facing an adjoining line of wire and the other side surface facing no adjoining line of wire, and the cover layer formed of insulating material is formed on the other side surface of the line of wire.
This construction can provide the result of protecting the lines of wire against an external physical force applied from a lateral side of the lines of wire, for example, when mounting and assembling the circuit board. Thus, the lines of wire can be prevented from being damaged by such an external force to thereby provide improved durability of the circuit board.
The present invention includes a circuit board comprising a base layer made of insulating material and a conductive layer formed on the base layer in the form of a specified circuit pattern, wherein top surfaces and side surfaces of lines of wire of the circuit pattern are covered with a cover layer formed of insulating material and land portions of the base layer extending between the lines of wire are not covered with the cover layer.
This construction enables the dielectric constant between the lines of wire to be reduced, as compared with the conventional type of circuit board, and thus enables the capacitance therebetween to be reduced, while it can provide the result of protecting the lines of wire further efficiently against an external physical force applied from the above and lateral sides of the circuit board, for example, when mounting and assembling the circuit board. As a result of this, the lines of wire can be prevented from being damaged by such an external force to thereby provide improved durability of the circuit board.
This circuit board of the present invention is suitably used as a suspension board with circuit. The circuit board used as the suspension board with circuit mounting thereon the magnetic head of hard disk drive enables a large volume of information to be read and written by the magnetic head to be transmitted at high speed.